How to scale dimensions proportionally to a specific size?
As far as I know it can't easily be done on a single atomic operation unfortunately, but I generally use the following workflow.

To scale uniformly with precise absolute dimensions, in the Properties Area of the 3D View just type in the exact absolute dimension you require on the single desired axis.

Now check the resulting scaling on that axis, Blender will have calculated the appropriate factor for that vector. You can now copy-paste the value with Ctrl+C, Ctrl+V to the other axis so they all remain uniform.

Apply the scale afterwards if you desire with Ctrl+A > Scale while in Object Mode.